Making a corrective lens for spectacles comprises, on the one hand, the optical design and shaping of the refractive faces of the lens and, on the other hand, the adaptation of the lens to the selected spectacle frame.
The present invention deals with the measurement, on the face of the wearer, of geometric/physiognomic parameters that give an account of the positioning configuration of the spectacles on the face of the wearer. These parameters are likely to be used in the two steps of making a corrective lens, in order for the lens to ultimately provide the corrective optical function for which it was designed and prescribed. They are notably the interpupillary distance, the height of the pupils of the eyes of the wearer relative to the bottom edge of the frame, and/or the pantoscopic angle formed by the general plane of the frame or of the lens relative to the vertical.
The looked-for geometric/physiognomic parameters are linked both to the geometry of the head of the wearer and the selected spectacle frame, as well as to the posture of the wearer.
As is known, it is possible to determine these parameters manually. For example, the measurement of the height of the eyes of the wearer relative to the bottom edge of the frame can be performed by the optician who observes the wearer from the front and makes a rough measurement, by means of a rule, of the distance between the pupil of an eye and the bottom edge of the presentation lens.
These parameters can also be determined from one or more captured images of the wearer's head.
However, whether manual or computerized, these methods for determining geometric/physiognomic parameters of the wearer suffer from a great inaccuracy. In practice, to perform an accurate measurement by rule or by processing captured images, it is necessary for the wearer's head to be in its natural posture at the time of the measurement or of the image capture.
This natural posture is also called anatomical posture or orthostatic position. In this natural posture, which will be defined in more detail later, the wearer holds his or her head straight and looks toward the distance, toward the horizon.
For reasons of speed of execution and comfort for the optician as for the wearer, the position of the wearer's head when the image is taken is not imposed.
However, even if the position of the wearer's head is not imposed, the wearer is not usually in the natural posture while the geometric/physiognomic parameters are being determined.
This is because the application of these methods for determining the geometric/physiognomic parameters entails bringing a measuring apparatus close, generally to less than a meter, to the wearer. The optician is also close in the case of a manual measurement.
The wearer then looks at this measuring apparatus or the optician who is then located in front of him or her during the measurement.
The proximity of the apparatus or of the optician causes the wearer, in practice, to bend slightly backward.
The wearer is then in a strained and unnatural posture. His or her gaze is not directed straight in front, to the horizon, but fixed on a close object.
Now, a deviation of 1 degree of inclination of the head in its sagittal plane relative to the anatomical posture introduces an error of 1 degree on the measurement of the pantoscopic angle and an error of 0.5 millimeter on the measurement of the heights of the eyes.
Moreover, determining said geometric/physiognomic parameters from captured images entails identifying, on the captured image, the image of at least one indexing element positioned on the face of the wearer and having at least one predetermined geometric characteristic.
It is then possible to determine the looked-for geometric/physiognomic parameter by comparing the geometric characteristic of the image of the indexing element and its corresponding real geometric characteristic.
However, such a method can be implemented only if the identification of the indexing elements on the captured image is possible.
This is not notably the case if the captured image is blurred or if it is overexposed or underexposed to the point that the image of the indexing elements cannot be identified.
In particular, when an image is captured using a device comprising a means for automatically setting the focal distance of acquisition of the image, it is possible that the image capture device will focus on a point of the image that does not form part of the wearer's head, which means that the wearer's head appears blurred on the captured image. Furthermore, when the wearer is positioned backlit relative to the image capture device, the captured image is often too dark to allow the image of the indexing element to be identified.
One solution to these various problems consists in manually setting the sharpness and the brightness of the captured image before each image capture. However, this presents the drawback of being long and tedious for the optician. Furthermore, it is not feasible in the context of continuous image acquisition in video mode.